The size effect of ferroelectric materials is one of the most important issues to develop next-generation dielectric devices. In this paper, we reviewed our studies on the size effects of barium titante (BaTiO₃) fine particles and ceramics. In the size effect of BaTiO₃ fine particles, the maximum of dielectric permittivity was observed at a particle size of around 140 nm, and was explained by the composite structure model including a gradient lattice strain layer (GLSL) having a very high permittivity. In contrast, the grain size effect of BaTiO₃ ceramics, where the permittivity showed a maximum at a grain size of about 1.1 µm, was interpreted as the superposition effect of domain-wall contributions and grain boundary effect.